The invention relates to the use of specific phosphonium salts as catalysts for isocyanate modification (oligomerisation or polymerisation), and to a process for the preparation of correspondingly modified isocyanates.
The oligomerisation or polymerisation of isocyanates, referred to here in summary as isocyanate modification, has been known for a long time. If the modified polyisocyanates contain free NCO groups, which may optionally also have been temporarily deactivated with blocking agents, they are extraordinarily high-quality starting materials for the production of a large number of polyurethane plastics and coating compositions.
A number of industrial processes for isocyanate modification have become established, in which the isocyanate to be modified, in most cases a diisocyanate, is generally converted by addition of catalysts and then, when the desired degree of conversion of the isocyanate to be modified has been reached, the catalysts are rendered inactive (deactivated) by suitable measures and the resulting polyisocyanate is generally separated from the unconverted monomer. A compilation of these processes of the prior art is to be found in H. J. Laas et al., J. Prakt. Chem. 1994, 336, 185 ff.
Compounds having an ionic structure have proved to be successful as modification catalysts, because they can be used in a very small amount and yield the desired result extremely quickly.
The possibility of using also tetraorganylphosphonium as cation to the anion having catalytic activity in respect of isocyanates, such as hydroxide, alkanoate, alkoxylate, etc., is generally known, although it is generally not explicitly given prominence as being particularly preferred, see: H. J. Laas et al., J. Prakt. Chem. 1994, 336, 185 ff.
Furthermore, the use of (hydrogen poly)fluorides, optionally also in the form of their phosphonium salts, for isocyanate modification is known inter alia from EP-A 962455, EP-A 962454, EP-A 896009, EP-A 798299, EP-A 447074, EP-A 379914, EP-A 339396, EP-A 315692, EP-A 295926 and EP-A 235388.
However, when the modification reaction is carried out, the tetraorganylphosphonium (hydrogen poly)fluorides of the prior art exhibit the disadvantage that occasionally, when they are used, the reaction can be maintained only by the continuous metered addition of catalyst, that is to say the decomposition of the catalyst in the isocyanate medium takes place unacceptably quickly in technical terms as compared with the modification reaction.